The present application claims priority to, and benefit under 35 USC xc2xa7119(a)-(d) of, the European Patent Application No. 00830616.9, filed Sep. 13, 2000.
1. Field of the Invention
The invention refers to devices for tensioning drive transmission means, such as chains. In particular reference will be made to chain transmission means on vehicles.
2. Description of Related Art
A timing system of an internal combustion engine can be controlled by means of a chain drive, in which a chain is wound on two or more sprockets, one of which is a driving sprocket and takes the motion from the crankshaft to transmit it to the camshaft.
Since for reasons of adjustment, wear on materials and take-up of slack it is often necessary to compensate for a certain looseness on the chain, the use of shoe tensioning devices is known to the art, in which a shoe is biased with an adjustable force against a run or branch of the chain.
Various means are known for biasing the tensioning shoe against the chain. Among these, the most frequently used are cylinder/piston tensioners, in which a fixed member of a cylinder/piston assembly (generally the cylinder) is mounted on the engine block and a movable member (generally the piston) is slidable with respect to the fixed members and acts against the shoe placed in contact with the chain, to tension it.
In these tensioning devices, the piston is pushed out of the cylinder, towards the shoe disposed against a run of the chain by the combined action of a spring and of pressurized oil fed into the cylinder chamber. Any slackening of the chain due to heating, wear and time is compensated by the piston extending out of the cylinder, under the action of said biasing means.
Said tensioning devices can be further divided into two categories: those that are disposed inside the engine block, and therefore require opening of the engine for fitting and, if necessary, for removal, and those of the so-called cartridge type, which are screwed into the engine block or the head from the outside, these latter devices undoubtedly being more convenient as far as ease of fitting and removal is concerned, in that opening of the engine is not necessary.
The invention refers in particular to this second type.
It sometimes happens, especially during starting of the engine, that the cylinder chamber is completely or partially emptied of oil. In this case, the force of the pressure spring alone may not be sufficient to keep the chain under control. Thus flapping of the chain is generated, which tends to push the piston inside the cylinder, until the oil in the cylinder reaches working pressure, which may take a few seconds. This flapping of the chain, besides producing an annoying noise, can in the long run cause damage to the whole chain drive assembly, including the tensioner.
In order to avoid this drawback, no-return devices for the piston have been proposed, which are easily applied in tensioning devices situated inside the engine block, where there are no particular problems of space. These devices normally consist of a rod provided with a rack located outside and parallel to the cylinder/piston assembly and engaged by a pawl disposed in the body of the tensioning device and elastically biased against said rack, which is allowed to slide only in the direction of outward travel of the piston and is pulled by the piston in this direction, when the latter protrudes from the cylinder because of slackening of the chain. The rod with rack, being prevented from returning inside the body of the tensioner, forms a stop means for the shoe that acts against the chain, thus preventing the above-mentioned flapping of the chain. In order not to make the system excessively rigid, the rod with rack engages with a shoulder of the piston situated at a certain distance (of the order of a few millimeters) from the free end of the piston, so as to allow a certain degree elasticity of the system, that is, a certain controlled return of the piston.
For a better understanding of the above described device of the prior art, reference can be made to FIG. 1 of the appended drawings.
In so-called cartridge type tensioning devices said solution is not practicable, in that in said devices it is not possible to dispose members that protrude beyond the outer profile of the cylinder. The maximum size of said devices is in fact given by the thread that screws into the threaded hole in the engine block.
For these cartridge type tensioners no-return devices for the piston have been proposed, which, however, prove excessively complicated, costly because of the considerable mechanical working they require, and not entirely reliable.
The object of the invention is to eliminate said drawbacks, providing a tensioning device for chain tensioners of the cartridge type, provided with a no-return device for the piston completely contained within the bulk of the cylinder.
Another object of the invention is that of providing such a cartridge tensioner, in which said no-return device for the piston is of simple and economical design, and is highly reliable.
A cartridge type tensioner for chain or belt transmission systems is provided. The tensioner includes a body or cylinder with an inner bore and a hollow piston axially slidable in the cylinder under the action of elastic biasing means acting between said cylinder and said piston and of pressurized fluid fed into said bore. The result is that an upper end of the piston acts on a movable shoe to tension a chain or belt. The tensioner further includes a device able to prevent uncontrolled return of said piston into the cylinder, characterized in that on a skirt of the piston at least one longitudinal rack is provided. The rack is engageable by a toothed pad which extends through an aperture formed in a wall of the cylinder and biased by an elastic means housed in a seat of the cylinder, without protruding from the outer profile of said cylinder.
Substantially, according to the invention, at least one set of teeth or rack, extending longitudinally of the piston and accessible through at least one window in the cylinder wall is provided, and in register with said window a toothed pad is disposed, able to engage with the teeth of the rack, so as to allow sliding thereof only in a direction of protrusion of the piston from the cylinder, said pad being elastically biased against the rack by the action of a spring.
The longitudinal extent of the pad is advantageously smaller than the corresponding extent of the aperture or window made in the wall of the cylinder, and the pad is suitably guided in a seat made in the wall of the cylinder, so as to allow the piston a certain play, and thus its possible controlled elastic return.
Alternatively the pad acting against the rack can be mounted without any possibility of axial sliding and a second small piston, axially protruding from the end of the main piston can be provided able to retract in the event of excessive force being exerted against it, thus maintaining the elastic characteristic of the piston.
Further characteristics of the invention will be made clearer by the detailed description that follows, referring to exemplary unrestrictive embodiments thereof, illustrated in the appended drawings, in which: